1. Technical Field
The present invention relates generally to a method and apparatus for generating terahertz waves using laser plasma.
2. Description of the Related Art
Terahertz waves are signals in the band between the band of microwaves and the band of far-infrared light waves in the electromagnetic spectra, thereby exhibiting characteristics of both microwaves and light waves.
Terahertz waves have very important usages in spectroscopy or in the acquisition of high-density material transmission images thanks to the characteristics thereof. However, the development of terahertz wave applications has been slow because of the problem with the generation of electromagnetic waves in the corresponding frequency band and the difficulty in the implementation of measurement technology.
However, in recent years, terahertz waves have been generated and measured by the combination of a laser having a pulse width of hundreds of femto-seconds and an opto-photoconductive material having a carrier life time shorter than several pico-seconds, with the result that a variety of technologies using the characteristics of terahertz waves have been developed.
Because terahertz waves can pass through materials though which microwaves and light waves cannot pass, various apparatuses using such characteristics of terahertz waves are now being developed.
Meanwhile, in the conventional art, methods of generating high-power terahertz electromagnetic waves by radiating high-power laser beams into solid media are currently used to generate terahertz waves.
Although terahertz waves are evaluated as next-generation waves capable of replacing X rays because they can be used to analyze organisms without damage, the use of high-power terahertz waves has not yet been completely established. Terahertz (THz) waves generated by the interaction between plasma and one or more laser beams attract a lot of attention because they can achieve high power and various attainable frequencies. The principal methods of generating THz waves are illustrated in FIGS. 2A to 2C.
First, FIG. 2A illustrates a 2-color method in which THz waves are generated by simultaneously radiating two low-power laser beams having different wavelengths into plasma. The generated THz waves have a field strength of several MV/cm. Among the various methods, the 2-color method generates the highest power THz waves and relatively wide THz frequencies.
Next, FIG. 2B illustrates a linear mode transformation method in which THz waves are generated by radiating a high-power laser beam into a region where plasma varies in density. The generated THz waves have a field strength of several MV/cm and a relatively wide range of THz frequencies.
Finally, FIG. 2C illustrates a Cherenkov wake method in which THz waves are generated by radiating a high-power laser beam into magnetized plasma. Since there is a limitation to the intensity of a magnetic field, the generated THz waves have a low power and a relatively narrow range of THz frequencies.
As described above, only the Cherenkov wake method can achieve a narrow range of THz frequencies. However, the Cherenkov method requires a high-power laser.